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I had a Lennox 2 stage furnace installed recently(G61) and am considering replacing an 8 year old AC unit( Bryant 10 seer). I am considering replacing it with a compatible Lennox unit but do not quite understand the advantages of a 2 stage unit or if it is worth the additional cost. The sales person did a load calculation and I need a 3 ton unit. I live in Atlanta with hot, humid summers. Thanks for your advice.

2 stage a/c unit

Well the advantages of a 2 stage a/c unit are basicly the same as a 2 stage furnace. On real hot days your 2nd stage would kick in and as you neared you set point temp you would go back in to 1 stage using less power. Basically the temperature curve would not be as broad. 2 stage units are more efficient.

2 stage units are much more efficient than single stage.
seer on the old lennox 2 speed was rated around 11 or 12 on HIGH speed, on low speed it was up around 23 or 24. unit runs most of the time on low speed to maintain house temp. comfort due to better dehumidification is a side benefit.

Some of the old two speeds operated on low compressor with high speed fan, high Seer and low/no latent removal on low speed. The new specs are for the same seer and sensible/latent ratio high or low speed. Advantage is less short cycling which provide slightly lower %RH during low a/c load. These systems are unable to provide <50%RH during low/no cooling load conditions. Looking at real comfort, humidity control during all the different a/c loads, total investment, and simple easy to maintain system, a straight high seer a/c and whole house ventilating dehumidifier is a better value. This combination provides ideal comfort at any temperature, fresh, filtered ventilation, and low cost operation.